1. Field of the Invention
The present invention relates to a laser irradiation apparatus used for crystallizing a semiconductor film. In addition, the present invention relates to a laser irradiation method and a method for manufacturing a semiconductor device with the use of the laser irradiation apparatus.
2. Description of Related Art
A thin film transistor using a poly-crystalline semiconductor film (poly-crystalline TFT) is superior to TFT using an amorphous semiconductor film in mobility by double digits or more, and thereby has an advantage that a pixel portion and its periphery driver circuit in a semiconductor display device can be integrally formed on the same substrate. The poly-crystalline semiconductor film can be formed over an inexpensive glass substrate by using a laser annealing method.
Lasers are generally classified into two types of a pulsed laser and a continuous wave laser according to the oscillation method. The output energy of the pulsed laser typified by an excimer laser per unit of time is higher by three to six digits than that of the continuous wave laser. Therefore, throughput can be enhanced by shaping a beam spot (a region irradiated by the laser light in fact on the surface of the processing object) into a rectangular spot having a length of several cm on a side or into a linear spot having a length of 100 mm or more through an optical system and by irradiating the laser light to the semiconductor film effectively. For this reason, the pulsed laser has become popular to be employed for crystallizing the semiconductor film.
It is noted that the term “linear” herein used does not refer to a line in a strict sense but to a rectangle (or an oblong) having a large aspect ratio. For example, the rectangular spot having an aspect ratio of 2 or more (preferably in the range of 10 to 10000) is referred to as linear. It is noted that the linear is still included in the rectangular.
However, the semiconductor film thus crystallized using the pulsed laser light includes a plurality of crystal grains assembled and the position and the size of the crystal grain are random. Compared to an inside of the crystal grain, a boundary between the crystal grains (crystal grain boundary) has an amorphous structure and an infinite number of recombination centers and trapping centers existing due to a crystal defect or the like. There is a problem that when a carrier is trapped in the trapping center, potential of the crystal grain boundary increases to become a barrier against the carrier, and thereby lowering a transporting characteristic of the carrier.
In view of the above problem, recently, attention has been paid to the technique of irradiating the continuous wave laser light to the semiconductor film. In this technique, the continuous wave laser is scanned in one direction so as to grow crystals continuously toward the scanning direction and to form a plurality of crystal grains including single-crystal grains extending long in the scanning direction. It is considered that this technique can form a semiconductor film having few crystal grain boundaries at least in a channel direction of TFT.
By the way, it is preferable that the absorption coefficient of the laser light to the semiconductor film is high because the higher the absorption coefficient is, the more effectively the semiconductor film can be crystallized. The absorption coefficient depends on the material and the like of the semiconductor film. In case of using a YAG laser or a YVO4 laser to crystallize the silicon film having a thickness from several tens to several hundreds nm which is generally employed for the semiconductor device, the second harmonic having a shorter wavelength than the fundamental wave is much higher in the absorption coefficient. Therefore, the harmonic is usually used in the crystallization process and the fundamental wave is rarely used.
However, the output power of the laser light converted into the harmonic is lower than that of the fundamental wave. Therefore, it is difficult to enhance the throughput by enlarging the area of the beam spot. Particularly, since the output power of the continuous wave laser per unit of time is lower than that of the pulsed laser, the throughput becomes lower. For example, when a Nd: YAG laser is used, the conversion efficiency from the fundamental wave (wavelength: 1064 nm) to the second harmonic (wavelength: 532 nm) is about 50%. Moreover, the nonlinear optical element converting the laser light into the harmonic does not have enough resistance against the laser light. For example, the continuous wave YAG laser can emit the fundamental wave having an output as high as 10 kW, while it can emit the second harmonic having an output as low as 10 W. Therefore, in order to obtain necessary energy density for crystallizing the semiconductor film, the area of the beam spot must be narrowed to approximately 10−3 mm2, and therefore the continuous wave YAG laser is inferior to the pulsed excimer laser in terms of throughput.
It is noted that in opposite ends of the beam spot in the direction perpendicular to the scanning direction, there is formed a region where the crystal grain is extremely small and where the crystallinity is inferior compared with the center of the beam spot. Even though a semiconductor element is formed in such a region, a high characteristic cannot be expected. Therefore, it is important to reduce the proportion of the region where the crystallinity is inferior in the whole region irradiated by the laser light in order to relax the restriction in the layout of the semiconductor element.
Moreover, in the surface of the region where a microcrystal is formed in the vicinity of the edge of the beam spot, there are formed concavity and convexity (ridge) having the height which is nearly equal to the thickness of the semiconductor film. Therefore, in the case of TFT for example, it is difficult to uniform the thickness of the gate insulating film formed so as to contact the active layer, and this makes it difficult to thin the gate insulating film. For this reason, there is a problem that miniaturization of TFT and the other semiconductor element is interrupted.